The present invention relates to cationic starch graft copolymers which are suitable for increasing the dry strength of paper, their use and their preparation.
The dry strength of paper covers various strength parameters which are determined on the dry paper, such as dry tensile strength, bursting strength, cleavage strength or strength in the z direction, stiffness, buckling resistance and surface strength (cf. “Dry Strength Additives”, Ed. W. F. Reynolds, TAPPI Press 1980, Atlanta). For increasing the dry strength, certain additives can be added to paper. The dry strength is determined primarily by hydrogen bridges within the paper sheet, in addition to the natural strength of the cellulose fibres themselves. Polymeric, hydrophilic additives which can form hydrogen bridges with the cellulose fibres and thus increase the contact area between the cellulose fibres have therefore proved useful. Typical examples of these are starch, galactomannans, polyacrylamides, carboxymethylcellulose and polyvinyl alcohol.
Additives for increasing the dry strength, i.e. dry strength agents, can be added to the paper pulp and/or applied to the surface of the paper. For use in the pulp, either cationic, self-retaining polymers are employed or anionic or amphoteric polymers are used in combination with a cationic fixing agent. In the case of surface use, the charge plays no role for the fixing, so that virtually uncharged polymers, such as polyvinyl alcohol, can also be used here. Starches and galactomannans which are used as dry strength agents in the pulp are usually cationically modified. Polyacrylamides are used in anionic, amphoteric or cationic form, amphoteric or anionic polyacrylamides usually being combined with a fixing agent when used in the pulp in order to achieve good retention in the paper.
U.S. Pat. No. 4,400,496 describes graft polymers based on starch and acrylamide, starch serving as the grafting base and the grafts either consisting only of acrylamide, i.e. being nonionic, or being composed of acrylamide and acrylic acid, i.e. being anionic. If they are added as dry strength agents to the paper pulp, such products must be combined with a fixing agent in order to ensure sufficient retention. EP-A 194 987 describes cationic starch graft polymers as paper dry strength agents, starch being used as grafting base and the grafts consisting of cationic polyacrylamide fragments. This is achieved by grafting on acrylamide together with a cationic vinyl monomer. The products described can be used without a fixing agent in the paper pulp since they have sufficient intrinsic retention owing to the cationic charge. A significant increase in the dry breaking strength is achieved thereby.